Grain Boundary Diffusion and Mechanisms of Creep of Nanostructured Metals

Comparative investigations of diffusion in coarse-grained (d = 20 μm), nanocrystalline (d = 0.04 μm) and nanostructured nickel (d = 0.3 μm) have been carried out in a temperature interval of 0.2–0.3 melting temperature. The reasons for difference of parameters of copper grain-boundary diffusion in the above materials are discussed. The effect exerted by grain boundary state and grain boundary diffusion fluxes of impurity on creep mechanisms of nanostructured nickel and copper in the temperature interval of 373–473 K have been studied. Significant change in the apparent creep activation energy under copper grain boundary diffusion fluxes is described as a consequence of different contribution of grain boundary sliding to overall deformation.     

Creep and diffusion parameters in submicrocrystalline metals

We have studied the creep of nickel and copper in a submicrocrystalline (SMC) state in a vacuum and in the presence of a diffusion contact with an impurity (Cu and Al, respectively). It is shown that a reduction of the resistance in the presence of a diffusion contact with an impurity is observed in the SMC materials in the temperature range 398 to 473 K. This range is 200 to 400 K lower than the corresponding range for coarse-grained material. It is shown that in this temperature interval the coefficients of grain boundary diffusion for copper in SMC nickel are 5 to 6 orders of magnitude larger than in the coarse-grained material. We propose that the reduction in the temperature for the manifestation of a creep activation effect in the presence of a diffusion contact with an impurity in SMC materials is caused by the increase in the diffusion permeability of the submicrocrystalline grain boundaries. Institute of Materials and Strength Physics, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 77–82, March, 1998.     

High-temperature creep of nickel under conditions of grain-boundary diffusion of impurities from the surface

Comparative investigations of the effect of diffusion streams of copper atoms (a weakly segregating impurity) and silver atoms (a strongly segregating impurity) from the surface in the high-temperature plastic deformation of nickel have been carried out. It has been established that in the high-temperature plastic deformation of nickel, when there are diffusion streams of copper and silver from the surface, there is a reduction in the creep resistance of nickel due to an increase in the contribution of grain-boundary slip to the overall deformation. Two stages, characterized by different values of the rate sensitivity factor m, are observed on the curve of the stress against the rate of deformation over a certain temperature range for each impurity. In the region of deformation rates of sec⁻¹, m≊0.2, and for sec⁻¹, m becomes less than 0.05. Institute of Physics of the Strength and Study of Materials. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 119–125, July, 1997.     

Laws of the deformation and failure of molybdenum in creep with grain-boundary activation by diffusional nickel fluxes

The diffusion of nickel from the surface of polycrystalline molybdenum influences the crack formation and plastic deformation in creep. Investigation shows that diffusional fluxes of nickel from the surface reduce the critical deformation at which the first cracks appear in molybdenum. The contribution of grain-boundary slip to the deformation of molybdenum in creep is considerably increased in the presence of nickel diffusion from the surface as a result of easier crack formation and grain-boundary activation.Institute of Strength Physics and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 16–21, July, 1992.     

On physical nature of creep acceleration effect in submicrocrystalline materials

An analysis of special features of manifestation of the creep acceleration effect in polycrystalline metals and alloys in the presence of grain-boundary diffusion fluxes of an impurity from an external medium is made. It is noted that in certain cases where the diffusion properties of sub-boundary regions differ greatly from those within the boundary and in the bulk of the grain (e.g., in materials with submicrocrystalline structure (grain size 0.1–0.5 µm) and/or in the case of diffusion along the grain boundary of a strongly segregating impurity), conventional theoretical methods of calculation of the temperature-temporal range of the occurrence of the above effect have to be corrected. Based on the analysis of the experimental data, feasible solutions to the problem are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 29–32, October 2004.     

Influence of grain sizes on the ionic conductivity and the chemical diffusion coefficient in copper selenide

Ionic conductivity and chemical diffusion coefficient have been studied for superionic polycrystalline Cu_1.75Se copper selenide within the temperature interval 300–500 K. An increase in ionic conductivity with an grain size increase is observed. In our opinion, this fact is caused by lower activation energy for the bulk diffusion than that for the grain boundary diffusion.     

Influence of copper solute on the creep rate sensitivities of aluminium

The presence of copper atoms as continuous networks at the grain boundaries of an aluminium-copper alloy has been considered not preventing the moving of dislocations during creep (or at least partially). The dislocations can bs absorbed by these boundaries and penetrate through them. That leads to changés of shape and structure of grains and also to the sliding of grains against each other. This was deduced from the accelerating increase in the sensitivity of the steady state creep rate to the applied stress of an aluminium 2·8 wt% copper alloy examined at wide range of temperatures (50–350 °C) and applied stresses (7–170 MPa). This rapid increase in the sensitivity parameter of the steady state creep rate occurs in Al-Cu alloys at quite higher ranges of applied stresses and may be attributed mainly to the contribution of the grain boundary movements to the creep strain.     

XPS study of low temperature diffusion in Au/Ni couples in the presence of arsenic

X-ray photoelectron spectroscopy, XPS, has been utilized to determine the low temperature grain boundary diffusion of nickel through gold thin films by means of surface accumulation. Surface accumulation is aided by a rapid oxidation of the nickel diffusant at the surface by arsenic vapor at very low (<10^-8 Torr) pressure causing a rapid accumulation of nickel arsenide product. The initial low temperature diffusion parameters of Au/Ni couples were measured in situ without interrupting the reaction-accumulation process. Accumulation and diffusion rates were obtained for temperatures ranging from 110 to 147° C. A discontinuity in an otherwise linear accumulation was observed after approximately 150 min. Evidence suggests that surface effects become rate limiting at this point.     

Machining of submicron structures on metals and semiconductors by ultrashort UV-laser pulses

Ablation of submicron structures on copper and silicon by short ultraviolet laser pulses (0.5–50 ps, 248 nm) is presented. Features like periodic line structures with a line-spacing below 400 nm, and holes with characteristic sizes well below 1 µm are produced on the sample surface by single laser shot exposure. The structures are projection printed by a Schwarzschild-objective (N.A.=0.4) in air environment. The morphology of ablation sites made with different pulse durations (0.5 ps, 5 ps, 50 ps) is discussed in terms of thermal diffusion effects.     

Characteristic features of diffusion-controlled processes in ordinary and ultrafine-grained polycrystaline metals

An analysis of manifestation of the effect of acceleration of creep in polycrystalline metals and alloys in the presence of grain-boundary diffusion fluxes of an impurity from an external medium (coating) is performed. The influence of the softened (due to impurity diffusion fluxes) layer thickness on the creep character in molybdenum is discussed for the case of its diffusion contact with nickel. An analysis of the effects of the regime of impurity grainboundary diffusion on the value of strain-rate sensitivity and on the behavior of grain-boundary migration as a function of external stress is made. Using submicrocrystalline nickel as an example, the influence of grainboundary state on properties of metallic materials in submicrocrystalline state is investigated. Direct measurements, theoretical calculations and indirect estimations of the differences in grain-boundary diffusion coefficients in coarse- and ultrafine-grained states are performed. The role of dispersion hardening in stabilizing the submicrocrystalline structure and improving the creep resistance is addressed, including the case where the grain-boundary fluxes of impurity atoms are forced from an external medium.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 49–64, August, 2004.     

Molecular dynamics and experimental studies on deposition mechanisms of ion beam sputtering

Molecular dynamics (MD) simulation and experimental methods are used to study the deposition mechanism of ionic beam sputtering (IBS), including the effects of incident energy, incident angle and deposition temperature on the growth process of nickel nanofilms. According to the simulation, the results showed that increasing the temperature of substrate decreases the surface roughness, average grain size and density. Increasing the incident angle increases the surface roughness and the average grain size of thin film, while decreasing its density. In addition, increasing the incident energy decreases the surface roughness and the average grain size of thin film, while increasing its density. For the cases of simulation, with the substrate temperature of 500 K, normal incident angle and 14.6 × 10⁻¹⁷ J are appropriate, in order to obtain a smoother surface, a small grain size and a higher density of thin film. From the experimental results, the surface roughness of thin film deposited on the substrates of Si(1 0 0) and indium tin oxide (ITO) decreases with the increasing sputtering power, while the thickness of thin film shows an approximately linear increase with the increase of sputtering power.     

Variation of apparent creep activation energy in plastic deformation of molybdenum in diffusion contact with nickel

The variation of the apparent creep activation energy as a function of the state of grain boundaries is investigated in the deformation of molybdenum in the presence of diffusion fluxes of nickel at the grain boundaries. It is shown that this energy varies in the same way as in the plastic deformation of classical superplastic materials.Physics Institute of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 110–113, May, 1993.     

Electric-field mediated nickel-induced nanocrystallization of?amorphous silicon thin films in the complete absence of external heating

DC electric-field mediated nanocrystallization of thermally evaporated silicon thin films with nickel as seed/cap layer has been attempted in complete absence of any external heat input. When 60 nm Si thin film coated onto 5 nm Ni thin film was treated by a direct current (DC) electric field (up to 3.3 kV/cm up to 5 minutes) after the deposition, amorphous silicon thin films became nanocrystalline (6–10 nm). Silicon nanograins (average diameter 90 nm) grow to larger sizes (average diameter 240 nm) with sharpening of grain size distribution. Huge grain growth (4-fold increase) has been observed when nickel was used as cap layer (5 nm Ni/60 nm Si). XRD data show the signature of nickel silicide formation on the surface in nickel cap layer case. Field treatment has changed the optical absorption edge (shifts left in nm units) and the refractive index of silicon thin film when nickel was used as under layer, and an almost negligible effect on the optical properties has been observed when nickel was used as cap layer.     

Growth of multi-walled CNTs emitters on an oxygen-free copper substrate by chemical-vapor deposition

Field emission properties of carbon nanotubes directly grown on a well-polished oxygen-free copper substrate by chemical-vapor deposition (CVD) were studied. Ni was sputtered on the copper substrate as catalyst, and the reactant gas was acetylene. From scanning electron microscopic and transmission electron microscopic images, the as-grown carbon nanotubes are seen to be bamboo structure with branches. Efficient field emission of CNTs is measured by a diode configuration, and the maximum current is 4.8 mA corresponding to a low electric field of 6.7 V/μm (the emission area is about 3.14 mm²). The diffusion between nickel and copper substrate is found to cause the loss of catalyst based on X-ray diffraction pattern of the surface of the substrate.     

Finite element modelling of creep cavity filling by solute diffusion

In recently discovered self healing creep steels, open-volume creep cavities are filled by the precipitation of supersaturated solute. These creep cavities form on the grain boundaries oriented perpendicular to the applied stress. The presence of a free surface triggers a flux of solute from the matrix, over the grain boundaries towards the creep cavities. We studied the creep cavity filling by finite element modelling and found that the filling time critically depends on (i) the ratio of diffusivities in the grain boundary and the bulk, and (ii) on the ratio of the intercavity distance and the cavity size. For a relatively large intercavity spacing 3D transport is observed when the grain boundary and volume diffusivities are of a similar order of magnitude, while a 2D behaviour is observed when the grain boundary diffusivity is dominant. Instead when the intercavity distance is small, the transport behaviour tends to a 1D behaviour in all cases, as the amount of solute available in the grain boundary is insufficient. A phase diagram with the transition lines is constructed.     

Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition

The graphene grain boundaries (GGBs) of polycrystalline graphene grown by chemical vapor deposition (CVD) typically constitute a major reason of deterioration of the electrical properties of graphene-based devices. To reduce the density of GGB by increasing the grain size, CVD growth conditions with a reduced CH₄ flow rate have been widely applied and, recently, electropolishing of copper (Cu) foil substrates to flatten the surface has been undertaken prior to graphene growth. In this study, we show that polycrystalline graphene layer grown on typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Statistical analysis of the grains of the graphene layers grown under different process conditions showed that SGRs (which form on Cu striations) limit the average grain size, the ability to control the grain size through adjustment of growth conditions, and global grain-size uniformity. Analysis showed that the surface-flattening process significantly improves grain-size uniformity, and monolayer coverage, as well as the average grain size. These results suggest that a process for flattening the surfaces of Cu substrates is critical to controlling the quality and uniformity of CVD-grown graphene layers for practical device applications.     

The interaction between Lateral size effect and grain size when scratching polycrystalline copper using a Berkovich indenter

It has been reported previously that, for single and polycrystalline copper (fcc), the indentation size effect and the grain size effect (GSE) can be combined in a single length-scale-dependent deformation mechanism linked to a characteristic length-scale calculable by a dislocation-slip-distance approach (X. D. Hou and N. M. Jennett, ‘Application of a modified slip-distance theory to the indentation of single-crystal and polycrystalline copper to model the interactions between indentation size and structure size effects,’ Acta Mater., Vol. 60, pp. 4128–4135, 2012). Recently, we identified a ‘lateral size effect (LSE)’ in scratch hardness measurements in single crystal copper, where the scratch hardness increases when the scratch size is reduced (A. Kareer, X. D. Hou, N. M. Jennett and S. V. Hainsworth ‘The existence of a lateral size effect and the relationship between indentation and scratch hardness’ Philos. Mag. published online 24 March 2016). This paper investigates the effect of grain size on the scratch hardness of polycrystalline copper with average grain sizes between 1.2 and 44.4 μm, when using a Berkovich indenter. Exactly the same samples are used as in the indentation investigation by Hou et al. (‘Application of a modified slip-distance theory to the indentation of single-crystal and polycrystalline copper to model the interactions between indentation size and structure size effects,’ Acta Mater., Vol. 60, pp. 4128–4135, 2012). It is shown that, not only does the scratch hardness increase with decreasing grain size, but that the GSE and LSE combine in reciprocal length (as found previously for indentation) rather than as a superposition of individual stresses. Applying the same (as indentation) dislocation-slip-distance-based size effect model to scratch hardness yielded a good fit to the experimental data, strongly indicating that it is the slip-distance-like combined length-scale that determines scratch hardness. A comparison of the fit parameters obtained by indentation and scratch on the same samples is made and some distinct differences are identified. The most striking difference is that scratch hardness is over four times more sensitive to grain size than is indentation hardness.     

Stoichiometry of BaTiO3 nanoparticles

Barium titanate nanoparticles with various nominal Ba/Ti ratios were prepared through direct synthesis from solution (DSS) and further annealed at different temperatures. Their deviation from stoichiometry was studied through XRD analysis, and a large deviation from stoichiometry has been observed. The grain size we studied ranges from 50 nm to 1 μm. For the as-prepared particles, the grain size is about 50 nm, and the maximum excess of Ti is over 15%. For the samples annealed at 800 °C, the grain size is increased to 100 nm, and the maximum excesses of Ba and Ti are 8 and 9%, respectively. The defects formed during synthesis and surface effect of nanoparticles are both estimated for their contributions to the deviation from stoichiometry, and the defects have been found to be mainly responsible for the large deviation observed.     

Rate of creep due to grain-boundary diffusion in polycrystalline solids with grain-size distribution

For steady-state deformation caused by grain-boundary diffusion, the macroscopic creep rate is analysed for a three-dimensional polycrystal consisting of space-filling grains, by taking into account the effects of diffusional interaction between grains, viscous grain-boundary sliding and grain-size distributions. For regular polyhedral grains, the grain–grain interactions increase the degree of symmetry of diffusional field, resulting in a decrease of the effective diffusion distance. Meanwhile, both the viscous grain-boundary sliding and the grain-size distribution are found to decrease the creep rate. At decreasing grain sizes, the influence of the viscous grain-boundary sliding becomes increasingly important, which explains the recent experimental observations that the creep rates of nanosized grains are much lower than those predicted by grain-boundary diffusion. On the effect of the grain-size distribution, the upper-bound and lower-bound creep rates are estimated.     

Synthesis of Y-doped BaCeO3 nanopowders by a modified solid-state process and conductivity of dense fine-grained ceramics

Powders of BaY_xCe_1 ? xO_3 ? δ (x = 0, 0.1 and 0.15) with specific surface area of 6–8 m²g^? 1 (BET equivalent particle size of 130–160 nm) were prepared by a modified solid-state route using nanocrystalline BaCO₃ and CeO₂ raw materials. These powders showed excellent densification at relatively low temperatures. Dense (96–97% relative density) ceramics with submicron grain size (0–4–0.6 µm) were obtained after sintering at 1250–1280 °C. Ceramics sintered at 1450 °C revealed only a moderate grain growth (grain size ≤ 2 µm), uniform microstructure and very high density (≥ 98%). The total conductivity of the submicron ceramics at 600 °C was comparable with the reference values reported in the literature, meaning that the high number of grain boundaries was not a limiting factor. On lowering temperature, the contribution of the blocking grain boundaries becomes progressively more important and the conductivity decreases in comparison to coarse-grained ceramics. Microscopic conductivities of grain interior and grain boundary are the same irrespective of grain size meaning that the different macroscopic behaviour is only determined by a geometric factor (a trivial size effect).